Logical pre- and post-selection paradoxes are proofs of contextuality

If a quantum system is prepared and later post-selected in certain states, "paradoxical" predictions for intermediate measurements can be obtained. This is the case both when the intermediate measurement is strong, i.e. a projective measurement with Luders-von Neumann update rule, or with weak measurements where they show up in anomalous weak values. Leifer and Spekkens [quant-ph/0412178] identified a striking class of such paradoxes, known as logical pre- and post-selection paradoxes, and showed that they are indirectly connected with contextuality. By analysing the measurement-disturbance required in models of these phenomena, we find that the strong measurement version of logical pre- and post-selection paradoxes actually constitute a direct manifestation of quantum contextuality. The proof hinges on under-appreciated features of the paradoxes. In particular, we show by example that it is not possible to prove contextuality without Luders-von Neumann updates for the intermediate measurements, nonorthogonal pre- and post-selection, and 0/1 probabilities for the intermediate measurements. Since one of us has recently shown that anomalous weak values are also a direct manifestation of contextuality [arXiv:1409.1535], we now know that this is true for both realizations of logical pre- and post-selection paradoxes.Comment: In Proceedings QPL 2015, arXiv:1511.0118

PBR, EPR, and All That Jazz

In the past couple of months, the quantum foundations world has been abuzz about a new preprint entitled The Quantum State Cannot be Interpreted Statistically by Matt Pusey, Jon Barrett and Terry Rudolph (henceforth known as PBR). Since I wrote a blog post explaining the result, I have been inundated with more correspondence from scientists and more requests for comment from science journalists than at any other point in my career. Reaction to the result amongst quantum researchers has been mixed, with many people reacting negatively to the title, which can be misinterpreted as an attack on the Born rule. Others have managed to read past the title, but are still unsure whether to credit the result with any fundamental significance. In this article, I would like to explain why I think that the PBR result is the most significant constraint on hidden variable theories that has been proved to date. It provides a simple proof of many other known theorems, and it supercharges the EPR argument, converting it into a rigorous proof of nonlocality that has the same status as Bell\u27s theorem. Before getting to this though, we need to understand the PBR result itself

Gamifying Quantum Theory

Over the past few years, the trend of gamification has gradually seeped into quantum research, education and outreach, so that we now have a critical mass of games based on quantum mechanics. I thought the time was right to discuss quantum gamification at the APS March meeting[.

Review of \u3cem\u3eThe Mathematical Language of Quantum Theory: From Uncertainty to Entanglement\u3c/em\u3e

A book review of The Mathematical Language of Quantum Theory: From Uncertainty to Entanglement by Teiko Heinosaari and Mario Ziman

Review of \u3cem\u3eComputing With Quantum Cats: From Colossus To Qubits\u3c/em\u3e and \u3cem\u3eSchrödinger’s Killer App: Race To Build The World’s First Quantum Computer\u3c/em\u3e

Book reviews of Computing With Quantum Cats: From Colossus To Qubits by John Gribbin and Schrödinger’s Killer App: Race To Build The World’s First Quantum Computer by John Dowling

Response to Griffiths

First of all, I would like to thank Prof. Griffith for his comments. The exchange has reminded me of the series of letters that appeared in Physics Today following the publication of an article by Chandralekha Singh, Mario Belloni, and Wolfgang Christian on improving the teaching of undergraduate quantum mechanics (see http://ptonline.aip.org/ journals/doc/PHTOAD-ft/vol_60/iss_3/8_1.shtml). In those responses, both Griffiths and Travis Norsen argued that students’ understanding of quantum mechanics would be vastly improved if they were taught more about the foundations of quantum theory, and I wholeheartedly agree with that sentiment. The thing is, Griffiths argued vociferously that this should be done by teaching students according to his approach, as outlined in his textbook Consistent Quantum Theory, whilst Norsen argued that it should be done by teaching students the de Broglie-Bohm theory, i.e. precisely the sort of theory that Griffiths argues strongly against in his response to my article

Review of \u3cem\u3eQBism: The Future of Quantum Physics\u3c/em\u3e

A review of QBism: The Future of Quantum Physics by Hans Christian von Baeyer

Review of Douglas Stone, \u3cem\u3eEinstein and the Quantum: The Quest of the Valiant Swabian\u3c/em\u3e

A book review of A. Douglas Stone\u27s Einstein and the Quantum: The Quest of the Valiant Swabian

Is a Time Symmetric Interpretation of Quantum Theory Possible Without Retrocausality?

Huw Price has proposed an argument that suggests a time symmetric ontology for quantum theory must necessarily be retrocausal, i.e. it must involve influences that travel backwards in time. One of Price\u27s assumptions is that the quantum state is a state of reality. However, one of the reasons for exploring retrocausality is that it offers the potential for evading the consequences of no-go theorems, including recent proofs of the reality of the quantum state. Here, we show that this assumption can be replaced by a different assumption, called λ-mediation, that plausibly holds independently of the status of the quantum state. We also reformulate the other assumptions behind the argument to place them in a more general framework and pin down the notion of time symmetry involved more precisely. We show that our assumptions imply a timelike analogue of Bell\u27s local causality criterion and, in doing so, give a new interpretation of timelike violations of Bell inequalities. Namely, they show the impossibility of a (non-retrocausal) time symmetric ontology